1. Field of the Invention
The present invention relates to a method and apparatus for achieving automatic loop gain adjustment in optical null spectrophotometers and, more particularly, to a method and means for automatically determining system loop gain requirements and setting the gain to the correct value for optimum response in a double beam, optical null spectrophotometer.
2. Description of the Prior Art
In a spectrophotometer of the double beam, optical null type, radiation from a source is split into two beams which are directed along sample and reference paths. The two paths are modulated by a suitable beam modulator which alternatively directs the radiation from the sample path and the radiation from the reference path to a recombining means and then to an electrical error signal generator. A sample to be analyzed is placed in the sample beam path and an attenuator for varying the intensity of the beam in the reference beam path is positioned therein, such attenuator being driven into and out of the reference beam path by a suitable servo motor. The error signal indicative of the difference between the radiation in the reference and sample beam paths is applied to a potentiometer or other means for adjusting the gain thereof. The gain adjusted error signal is coupled to the attenuator servo motor which moves the reference beam attenuator until the error signal is reduced to zero. The position of the reference beam attenuator may then be used to provide a measure of the sample content. Ordinarily, the wavelength of the radiator source is scanned over a range by a monochromator during the measurement to provide a spectrum measure of the sample.
The sensitivity and accuracy of such a spectrophotometer is affected by variations in loop gain of the attenuator servo loop which includes the optical path, the electrical circuitry, and the servo motor. If the loop gain is set too high, oscillation of the attenuator servo motor occurs. If the loop gain is set too low, the response of the servo motor is too slow. For this reason, the before-mentioned gain adjusting means has been included for controlling loop gain.
U.S. Pat. No. 3,790,283 describes one way of simplifying the gain setting operation in a manually controlled spectrophotometer. According to this patent, the reference beam path is closed completely by switching a large unbalancing signal into the amplifier which drives the attenuator servo motor and the gain control is then adjusted to produce a predetermined level of voltage at the input to the amplifier as measured by a DC voltmeter. However, a limitation of this system is that the reference beam attenuator must initially be at the position which is nominally designated as 100% transmission for the reference voltage level to be correct, a condition which is hard to achieve under actual analytical conditions.
Furthermore, the system of this patent requires the gain control to be adjusted to a nominal value for all conditions. While a nominal value of gain can be used for routine work, it is frequently necessary, in normal analysis, to attenuate the reference beam to achieve a desired output on highly absorbing samples or when differential analysis is being performed. In the former case, a beam attenuator in addition to the variable reference beam attenuator is positioned in the reference beam path whereas in the latter case, a reference sample is positioned in the reference beam path in addition to the variable reference beam attenuator. In either case, the amount of energy conducted to the electrical error signal generator is substantially reduced. At these times, it is necessary to increase the system gain to compensate for the losses in the reference beam energy.
In such cases, it has been proposed to utilize a small unbalancing signal to deflect the reference beam attenuator from its balanced position, the amount of deflection being inversely proportional to the system gain. With the unbalancing signal added, the change in the servo motor position is sensed to see whether the servo motor position changes by some predetermined amount indicative of the correct gain setting. If there is a difference between the actual change and the desired change, the loop gain may be manually adjusted and the procedure repeated until the servo motor position change is exactly the predetermined amount indicative of the correct gain setting.
The advantage of this method is that it is usable with a reference beam attenuator in any position where it may be deflected by the nominal amount without contacting the mechanical limits of travel. This method, while suitable for manual adjustment, is time consuming and requires a number of settings and repeated approximations to reach the desired optimum setting when attempting to automate the procedure.